Optical exposure apparatus for forming an alignment layer

ABSTRACT

A system for processing a multilayer liquid crystal film display material, with multiple irradiation apparatus ( 60 ) for applying a zone of polarized UV irradiation onto a substrate fed from a web ( 16 ) with incident light at a desired angle. Each irradiation apparatus ( 60 ) includes a UV light source ( 64 ), and one or more optional filters ( 82 ). A polarizer  90  is provided, sized and arranged to polarize light over the web ( 16 ) as it moves. The irradiation apparatus ( 60 ) employs an array of louvers ( 81 ) and/or a prism array ( 72 ). One irradiation apparatus ( 60 ) irradiates a first LPP1 layer ( 22 ) at a 0-degree alignment, in the web movement direction, the other irradiation apparatus ( 60 ) irradiates an LPP2 layer ( 26 ) with an orthogonal 90-degree alignment.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. 10/194,750, filed Jul. 12, 2002, entitled APPARATUS AND METHODFOR IRRADIATING A SUBSTRATE, by Leidig et al., the disclosure of whichis incorporated herein.

FIELD OF THE INVENTION

This invention generally relates to apparatus and methods for processingsubstrate materials using light exposure energy and more particularlyrelates to apparatus and methods for alignment of a liquid crystaldisplay compensation film provided as a web-fed substrate.

BACKGROUND OF THE INVENTION

Current rapid expansion in liquid crystal display (LCD) applications islargely due to improvements in display performance. High contrast, goodcolor reproduction, and stable gray scale intensities are importantattributes for electronic displays that employ liquid crystaltechnology. With respect to contrast, a primary constraint with liquidcrystal displays is the propensity for light leakage in the dark or“black” pixel state. Furthermore, the leakage and hence contrast of aliquid crystal display are also dependent on the angle from which thedisplay screen is viewed. Typically, optimum contrast is obtained onlywithin a narrow viewing angle, centered about the normal incidence tothe display, and falls off rapidly as the viewing angle is increased. Incolor displays, light leakage not only degrades the contrast, but alsocauses undesirable color or hue shifts, degrading color reproduction. Inaddition to black-state light leakage, viewing angle constraints fortypical twisted nematic liquid crystal displays are exacerbated by ashift in the brightness-voltage response as a function of viewing angle,due to the inherent optical anisotropy of the liquid crystal material.

Thus, one of the major factors that determine the quality of LCDdisplays is the viewing angle characteristic, which describes the changein contrast ratio relative to different viewing angles. It is desirablethat contrast be maintained over a wide range of viewing angles, a knownshortcoming with liquid crystal display devices. One way to improve theviewing angle characteristic is to insert a compensator (also referredas compensation film, retardation film, or retarder) with proper opticalproperties between the polarizer and liquid crystal cell, such asdisclosed in U.S. Pat. Nos. 5,583,679; 5,853,801; 5,619,352; 5,978,055;and 6,160,597. Compensation film according to U.S. Pat. Nos. 5,583,679and 5,853,801, based on discotic liquid crystals which exhibit negativebirefringence, is widely used. This film offers improved contrast overwider viewing angles; however, it suffers larger color shift for graylevel images, compared to a compensator made of liquid crystallinematerials having positive birefringence, according to Satoh et al.“Comparison of Nematic Hybrid and Discotic Hybrid Films as Viewing AngleCompensator for NW-TN-LCDs,” SID 2000 Digest, pp. 347-349, (2000). Toachieve comparable contrast ratio while reducing color shift, onecompensation film solution uses a pair of liquid crystal polymer films(LCP), having orthogonally crossed optical axes, disposed on each sideof a liquid crystal cell, as discussed by Chen et al. “Wide ViewingAngle Photoaligned Plastic Films,” SID 99 Digest, pp. 98-101 (1999).This paper states that “since the second LPP/LCP retarder film is coateddirectly on top of the first LCP retarder film, the total thickness ofthe final wide-view retarder stack is only a few microns thin.” Althoughsuch a method provides a very compact optical component, it is difficultto manufacture a compensation film having two LCP layers whose opticalaxes are orthogonally oriented. This is a particular challenge where thefilm substrate is web-fed, such as in a continuous, roll-to-rollmanufacturing process.

In processing liquid crystal compensation films, photo-alignment methodsare recognized to have advantages over earlier rubbing alignmentmethods. Using photo-alignment, a thin alignment medium, typicallylinear photo-polymerization media (LPP) is applied to a substrate and isthen irradiated, typically using UV light, to provide a directionalalignment bias. There are a number of photo-alignment methods, based ondifferent photoreaction processes. In general, a photo-alignment methodmay be one of three basic types:

-   -   (1) Isomerization, as disclosed in U.S. Pat. No. 4,974,941, is a        reversible process using laser light irradiation in which a        monomer or single molecule is aligned using        cis-trans-isomerization effects;    -   (2) Photo-dimerization, as disclosed in U.S. Pat. No. 5,602,661        employs photo-induced orientation and dimerization of polymer        side-chains, including cross-linking; and    -   (3) Photo-dissociation uses light to anisotropically alter an        alignment medium such as polyamic acid or polyamide or copolymer        comprised of amic acid and imide.

In one promising photo-dimerization method, discotic liquid crystalstructures within a liquid crystal polymer (LCP) layer are applied overan LPP layer to take the preferred alignment direction. Most solutionsfor photo-alignment using this method direct collimated polarized UVlight, at an oblique angle, onto an alignment LPP substrate to alignpolymer molecules in a desired direction that provides a pretilt for asubsequently applied LCP layer containing liquid crystal structures. Ithas been found that, for suitable performance, only a fraction ofmolecules in the LPP alignment layer need to be photopolymerized.Typical LCP media include diacrylates and diepoxides and similarcross-linkable liquid crystalline materials. LCPs may have inherentpositive optical anisotropy, such as with diacrylates, or negativeanisotropy and weak biaxial properties, such as with discotic liquidcrystal materials.

A number of different photo-alignment media and techniques have beenused to provide the necessary pretilt for different types of liquidcrystal display media. For a suitable class of LPP media, opticalapparatus that provides irradiation for alignment must meet thefollowing criteria:

-   -   1. Exposure levels of 10-15 mJ/cm², nominal.    -   2. Narrow range of wavelengths. The exact range that is suitable        for alignment irradiation depends on the material. UV-B (280-320        nm) is the preferred range for many types of alignment        substrate. Some wavelengths are preferably rejected in order to        minimize unwanted effects on alignment or undesirable        temperature effects. Rejection of unwanted wavelengths is        especially important for efficiency in a roll-to-roll        manufacturing apparatus in which a web of substrate traveling at        hundreds of feet per minute is processed. At such high speeds,        the necessary increase in radiation at desirable wavelengths can        easily bring with it an increase in undesirable radiation levels        from other parts of the spectrum. For example, UV light is        efficiently produced by a class of lamps that excite mercury or        ion-doped mercury molecules. Such lamps typically generate UV-C        (200-280 nm), UV-B (280-320 nm), UV-A (320-400 m), visible        light, and infrared light. For an embodiment where UV-B is        chosen as the preferred spectral range, it would be desirable to        limit the irradiance and total exposure on the web from other        parts of the spectrum.    -   3. Uniform exposure dosage. Exposure dose is expressed in terms        of energy per unit area. It has been found that dosage levels,        alternately termed exposure levels, can provide acceptable        alignment results even where dosage varies by as much as +/−50%        across the irradiated surface area in some applications.        However, reasonable compensation for dosage uniformity helps to        obtain uniform alignment results, minimizing intensity level        variations between levels at the middle of a substrate and at        substrate edges.    -   4. Uniform direction of polarization. It does not appear to be        important that the applied alignment radiation be highly        polarized. However, for a class of LPP materials, best results        are achieved when the exposure radiation has a highly uniform        direction of polarization. For maintaining a high standard of        quality and uniform alignment, it is preferable to provide a        consistent direction of polarization, varying no more than 1        degree over the full irradiated surface. Of course, the problem        of maintaining this tolerance for directional uniformity of        polarization is accentuated when irradiating a large scale        surface.    -   5. Oblique incident angles for pretilt. Typically, some        deviation from normal incidence to the media is required in        order to provide the necessary pretilt angle to the LPP        material. For most applications, a broad range of incident        angles, such as over a 10-70 degree range, is permissible. We        will refer to this illumination as illumination with        predetermined inclination where it is understood that the        inclination refers to the average angle of the multiplicity of        incident angles rather then a single angle of illumination.

There have been some conventional systems developed that generally meetmost of requirements 1-5 above for irradiating alignment media on asmall scale. However, it can be appreciated that these requirementsbecome particularly difficult to meet as the irradiated surface area, orexposure zone, increases. Conventional solutions are as yet poorlysuited to the demands for efficiently irradiating a web-fed substrate,where the substrate is moved past the irradiation device at productionspeeds and the web width exceeds 1 m.

In addition to the requirement for large scale photo-alignmentprocessing, there is also a need to provide a film having compositeLPP/LCP structures in which two alignment surfaces have been treated sothat their respective optical axes are close to 90 degrees, that is,orthogonal, with respect to each other. Conventional approaches have notyet provided a suitable solution for achieving this with a web-fedmedia.

Among proposed prior art solutions for photo-alignment are a number ofscanning solutions:

-   -   U.S. Pat. No. 5,889,571 discloses an irradiation device for        scanning linearly polarized UV across a substrate to achieve        alignment layer uniformity. U.S. Pat. No. 5,889,571 emphasizes        the importance of oblique radiation. This solution is best        suited to a substrate provided in sheet form rather than to a        substrate continuously fed from a web.    -   U.S. Pat. No. 6,295,110 discloses a laser light-based system for        applying polarized UV radiation across a substrate.    -   Designed for substrates having a diagonal in the range of about        10 inches or slightly larger, the U.S. Pat. No. 6,295,110        solution provides two-dimensional irradiation over an area that        exceeds the size limit for the type of optical radiation        employed. However, there are practical limitations in scaling        this type of solution to suit a web-fed substrate having a width        dimension of lm or larger.

It has been noted that high irradiance conditions would be beneficialfor use in high-speed roll-to-roll manufacturing apparatus, particularlywhere it is desirable to provide a compact system. Peak irradiance onthe web in such environments could range from approximately 50milliwatts/cm² to values of several hundred milliwatts/cm². This meansthat average irradiance on any polarizer would be much higher. Withirradiance over ranges such as might be supplied using a medium pressurelong-arc Mercury lamp at power levels in the 100-600 W range,conventional, resin-based polarizers would not be well-suited. Forexample, this type of irradiation exceeds the practical working range ofpolarizers such as HNP′B-Linear Polarizer from 3M (St. Paul, Minn.).Sheet polarizers are not generally capable of handling higherirradiation levels and may quickly deteriorate over a prolonged exposureperiod. With this limitation in mind, prior art solutions for providingpolarized irradiation over a large area include the following:

-   -   U.S. Pat. No. 6,190,016 discloses an irradiation apparatus using        an oval focusing mirror, integrator lens, and polarizer disposed        at various points in the optical system. U.S. Pat. No. 6,190,016        emphasizes the value of collimated light, incident to a        polarizer, to improve polarization performance. The use of        Brewster plate polarizers for large scale surfaces is disclosed.    -   U.S. Pat. No. 5,934,780 discloses an exposure apparatus using a        UV light source having an oval focusing mirror, where the        apparatus includes an integrator lens, polarizer, and        collimation optics. Brewster plate polarizers are used in the        preferred embodiment. This type of solution may work well for a        substrate up to a certain size. However, there are practical        size limitations that constrain the use of Brewster plate        polarizers for large substrates. Similarly, EP 1 020 739 A2        discloses a modified Brewster plate arrangement As a variation        on Brewster plate polarizers, EP 1 172 684 discloses a modified        V-shaped Brewster's angle arrangement. However, similar weight        and size constraints also limit the feasibility of this type of        solution.

U.S. Pat. No. 6,292,296 discloses a large scale polarizer comprising aplurality of quartz segments disposed at Brewster's angle, used forsystem that irradiates using UV. However, such an arrangement would bevery costly and bulky, particularly as a solution for a web-fed exposuresystem with a large irradiation area.

As the above-noted patent disclosures show, irradiation apparatusdesigned for large exposure zones have employed sizable polarizationcomponents, typically quartz or glass plates disposed at Brewster'sangle. Hampered by the relative size and weight of these polarizers,such irradiation apparatus are necessarily less efficient in deliveringlight energy to the exposure surface. Moreover, conventional polarizersusing Brewster plates or interference polarizers based on Brewster'sangle principles also exhibit a high degree of angular dependency. Thatis, incident light must be substantially collimated in order to obtain auniform polarized light output.

Significantly, Brewster plate polarizers such as those shown in the U.S.Pat. No. 5,934,780 and U.S. Pat. Nos. 6,061,138 and 6,307,609 are notoptimal for providing a uniform polarization unless highly collimatedlight is used. With respect to an irradiated surface, the principal axisof polarization of the Brewster plate polarizer is uniform only when theplane of the Brewster plates is within a very limited range of angles.Otherwise, the Brewster plate polarizer does not have a well-defined,uniform principal axis of polarization. With Brewster plate polarizers,the direction of polarization is dependent upon the angular direction ofincoming light. For each beam direction, a specific local coordinatesystem, aligned with the meridional plane containing incident andoutgoing beams, is established at the point of incidence, as is shown inFIGS. 16 a and 16 b. Thus, when there are several incoming beams atdifferent angles, the Brewster plate polarizer correspondingly providesmultiple polarization directions, that is, multiple polarization axes.Moreover, the Brewster plate polarizer operates in one direction only;it would not be practical to use Brewster plate polarizers for achievingorthogonal polarization of multiple LPP alignment layers on a web-fedsubstrate. Instead, in order to provide orthogonal alignment ofoverlapping LPP layers, it would be necessary to expose individual cutsheets of media, rotating the media to obtain orthogonal exposure.Brewster plate solutions are not compact or practical for use withlong-arc lamps, particularly where orthogonal exposure directions mustbe obtained.

Referring to FIGS. 16 a and 16 b, there is shown, for a Brewster platepolarizer 132, how a principal axis of polarization 126 varies withincident beams 124 at different angles. As is shown in these figures, ameridional plane 130 is defined by incident beam 124, a reflected beam120, and a transmitted beam 122. Principal axis of polarization 126 hasa variable angle ψ₁ or ψ₂ relative to a reference direction 128depending on the incident angle of incident beam 124. Viewedgeometrically, a tilt of meridional plane 130 results in a change toprincipal axis of polarization 126.

In contrast, conventional sheet polarizers have the property ofproviding a uniform principal axis of polarization for light from withina range of incident angles. Sheet polarizers are also capable of beingrotated to allow orthogonal alignment exposure such as would be requiredin a continuous web-based manufacturing process. However, sheetpolarizers are not robust under conditions of high UV light irradianceand would deteriorate rapidly. Thus, it can be seen that it is difficultto obtain efficient polarization of UV-B light (280-320 nm) atrelatively high irradiance levels and for incident light at relativelywide angles of incidence using conventional polarization components andtechniques.

Conventionally, wire grid polarizers have been used in infrared andlonger-wavelength applications. More recently, wire grid polarizers havebeen developed for use with visible light, as disclosed in U.S. Pat.Nos. 6,234,634 and 6,243,199. Although the concept of wire gridpolarizers for UV applications had been experimentally demonstrated in1983 (see Sonek et al. “Optical polarizers for the ultraviolet” ApplOpt. 22, pp. 1270-1271; where evaporated aluminum was spaced at 115 nmon quartz substrate to cover a wavelength range of approximately 200-800nm), only recently have wire grid polarization devices been commerciallyavailable for use with light in the UV range. Wire grid polarizers haveinherent advantages in high-heat and high-irradiance applications whereconventional sheet polarizers would not be suitable. Wire gridpolarizers are also inherently less angularly dependent than other typesof polarizers, particularly Brewster plate and interference typepolarizers. Advantageously, wire grid polarizers have a low dimensionalprofile, allowing them to be used to replace sheet polarizers wherespace along the optical axis may be minimal. In addition, wire gridpolarizers exhibit favorable response, similar to that available withsheet polarizers as noted above, with respect to principal axis ofpolarization. As is shown, for example, in FIGS. 16 c and 16 d, wiregrid polarizers 134 provide a principal axis of polarization 126 that isfairly uniform with respect to a reference direction 128 when incidentbeams 124 have a range of incident angles. This capability means, forexample, that wire grid polarizer 134 can be tilted with respect toincident beam 124 without a corresponding change in principal axis ofpolarization 126. In this way, principal axis of polarization 126 ofwire grid polarizer 134 is independent of the angle of incidence ofincident beams 124, over a broad range of angles. However, wire gridpolarization devices are not dimensionally scaled to suit therequirements of applying polarized light over a large exposure zone.

Alternatively, the Beilby-layer polarizer, commercially available fromSterling Optics (Williamstown, Ky.), has desirable properties forefficiently polarizing light in the UV spectrum. This type of polarizeruses an azo-dye applied and fixed to a uni-directionally polished plateof fused silica. The subsequent angular acceptance angle for theBeilby-layer exceeds that of either commercially available interferencefilters, Brewster plates, or UV wire grid polarizers, and surfaceresilience to high heat or irradiance is superior to that of resin-basedsheet polarizers. The Beilby-layer polarizer also exhibits a lowdimensional profile and a favorable response with respect to principalaxis of polarization.

A number of different types of light sources for photo-alignment havebeen disclosed, for example:

-   -   WO 00/46634 discloses a method for alignment of a substrate        using an unpolarized or circularly polarized source, applied in        an oblique direction.    -   U.S. Pat. No. 4,974,941 discloses alignment and realignment,        preferably using a laser source.    -   U.S. Pat. No. 5,389,698 discloses use of linearly polarized UV        for photopolymer irradiation. Similarly, U.S. Pat. No. 5,936,691        discloses use of linearly polarized UV for photopolymer        irradiation, with the UV source positioned close to the        substrate surface.

As noted above, the use of collimated or substantially collimated lightis, in large part, dictated by polarizer characteristics. In relatedexposure processing applications, collimated light is consideredadvantageous, as in these examples:

-   -   U.S. Pat. No. 5,604,615 and EP 0 684 500 A2 disclose forming an        alignment layer by directing collimated UV through slits in a        photomask.    -   In a related curing application, U.S. Pat. No. 6,210,644        discloses directing UV through slatted collimator for curing        resin.

U.S. Pat. Nos. 6,061,138 and 6,307,609 disclose a method and apparatusfor alignment using exposure radiation that is “partially polarized” and“partially collimated.” By “partially polarized,” this disclosureidentifies a broad range of S:P values from 1:100 to 100:1 withpreferred range from 0.5:1 to 30:1. By “partially collimated” thesedisclosures identify a broad range with a divergence, in one direction,greater than about 5 degrees and less than about 30 degrees. The use ofsuch broad ranges simply seems to indicate that some significant degreeof variability is acceptable for both polarization and collimation.Indeed, in practice, most polarizers work within the broad range statedin U.S. Pat. No. 6,061,138, particularly over sizable exposure zones. Asis generally well known and shown in the disclosure of U.S. Pat. No.6,190,016, some degree of collimation is needed simply for consistentcontrol of polarization. Partial collimation, over the broad rangesstated in U.S. Pat. No. 6,061,138 occurs when light simply passesthrough an aperture and is not otherwise blocked, focused, projected, ordiffused. Baffles or apertures that block stray light necessarilyperform “collimation” within the ranges given in the U.S. Pat. No.6,061,138. Earlier work, disclosed in U.S. Pat. No. 5,934,780 similarlyshows use of partially collimated light having relatively poorpolarization and the use of relatively high incident angles for exposureenergy, covering the ranges specified in the U.S. Pat. No. 6,061,138.Another earlier patent, EP 0 684 500 A2, states that collimation of theirradiating polarized light beam is preferable, but does not requirecollimation.

Thus, prior art seems to indicate that collimation, considered byitself, is not as important as other characteristics of exposureradiation. Certainly, some degree of collimation is inherently necessaryin order to efficiently collect and direct light onto a substrate,taking advantage of the light emitted in all directions by using devicessuch as using reflective hoods, for example. As is noted above, somedegree of collimation is necessary for polarizing light, sincepolarization devices are not typically equipped to handle widevariations in incident light divergence. But, taken in and of itself,collimation may have secondary importance relative to other propertiesof the exposure light.

In contrast, maintaining a consistent polarization direction orazimuthal angle appears to be very important for obtaining good results.The direction of polymerization or selection for LC alignment materialsclosely corresponds to the direction of polarization. In fact, there isevidence that partial polarization, as suggested by U.S. Pat. No.6,061,138 and as exhibited in earlier work disclosed by Schadt et al.“Surface-Induced Parallel Alignment of Liquid Crystals by LinearlyPolymerized Photopolymers” Japanese Journal of Applied Physics, Vol. 31,1992, pp. 2155-2164, appears to be acceptable, provided that aconsistent direction of polarization is maintained. The disclosure ofU.S. Pat. No. 5,934,780 emphasizes the importance of this direction ofpolarization. It has been shown that optimal results are obtained overthe exposure zone when the exposure energy is somewhat uniformlydistributed and when the direction of polarization is tightly controlledto within about 1 degree.

As is shown in the prior art solutions cited above, achievingpolarization over a broad exposure zone, with a tightly controlleddirection of polarization, is particularly difficult with high intensityTV-B radiation. It is difficult to obtain a UV-B source that providespolarized UV-B light at reasonable cost. Moreover, high heat andirradiance requirements place considerable demands on filtering andpolarization components. Conventional resin-based sheet polarizers areunlikely to withstand the elevated irradiance and high heat conditions.Brewster plates and interference filters can withstand these conditionsbut have size and weight disadvantages as well as acceptance angleconstraints.

As a further complication, controlling the intensity of radiation energyhas been proven to be difficult to achieve and maintain on a substrate.Obtaining a uniform dosage distribution from a light source requires ameans for redirecting the energy applied so that the intensity at theedges of the illuminated surface is not appreciably different from thatat the center of the surface.

While tolerances may not be critical, some reasonable degree ofuniformity appears to be desirable. Dosage is the product of irradianceand exposure time. If the substrate to be exposed is on a moving web,the dosage is an integration of the dosage received along the directionof motion over the time of exposure. The maximum permissible dosagevariation over the substrate depends on the resultant properties desiredover specific spatial distances. Differences in dosage applied to an LPPlayer will subsequently influence alignment of the adjacent LCP layer.Reasonable compensation for dosage uniformity helps to obtain uniformalignment results. Dosage uniformity may be slowly varying across largespatial distances; for example, as would be typical for a single lamplocated above and near the center of a substrate, dosage would be at amaximum near the middle of the substrate and decrease monotonicallytowards the outer substrate edges. Alternatively, other equipmentdesigns that include other reflecting, absorbing, refracting anddiffracting structures disposed between the lamp and substrate mayimpart changes in dosage that vary non-monotonically from the center toedge of a substrate; variations in dosage may vary over small patches orover small strips. These variation may be even more important to controlthan the slowly varying change because the slope of the change may belarge.

In applications where light throughput is important, standard diffusersare inefficient at correcting the problem of dosage uniformity becauseof the wide dispersal of the incident radiation and diminishedthroughput relative to polished optical surfaces. A standard diffuser istypically a piece of transparent material ground, etched or molded on atleast one side with small features that refract and scatter light into ahemisphere. A significant fraction of the light incident on a standarddiffuser is typically reflected and scattered back uselessly.Additionally, the wide dispersal of incident radiation into a hemisphereis at odds with the goal of maintaining an average angle of inclinationfor aligning an LPP layer.

Alternatively, there are other methods that partially overcome some ofthe aforementioned difficulties of maintaining the average angle ofinclination of the incident light and reducing the fraction of lightthat is uselessly reflected and back-scattered. One method is to use a“fly's eye” optical integrator, such as those employed in solarsimulators like those from Oriel Corporation (Stratford, Conn.). Also,the long-known method of using an integrating bar for mixing light canbe applied. For example, U.S. Pat. No. 1,917,360 discloses the use of anintegrating bar with diffuser for a film printer. More recentapplications of integrating bars, as exemplified by WO 01/80555 A1, donot use diffusers on the integrating bar and consequently maintainbetter directionality of the light cone. Light homogenizers like “fly'seye” and integrating bars are typically used in projection systems wherethe size of the device is not the foremost consideration; systems withan integrating bar or “fly's eye” lens typically scale with the maximumdimension of the substrate to be irradiated. Consequently, the approachof using a single projection device employing a “fly's eye” opticalintegrator or integrating bar is less amenable to being adapted toirradiate and align increasingly large substrates. Using a single smallinstrument that scans a large substrate raises problems with therapidity of alignment. Using multiply operated projection instruments toexpose a substrate in separate swaths or small patches increases thelogistical problems of aligning and managing multiple projection unitsto achieve consistent uniformity of exposure.

Cost-effective mass manufacture of LC material requires high throughput.This necessitates using sufficient intensity levels, consistentlyapplied to a material that is exposed and cured at fast speeds. Althoughconventional solutions provide some capability for high-volume web-fedmanufacture, there is clearly room for improvement in processing speed,cost, and quality over prior art approaches.

It can be appreciated that there would be benefits to manufacturingapparatus and methods for fabrication of a compensation film havingorthogonally disposed optical properties. Such a film would enhance theviewing angle performance liquid crystal displays, for displaytechnologies including twisted nematic (TN), super twisted nematic(STN), optically compensated bend (OCB), in plane switching (IPS), andvertically aligned (VA) liquid crystal displays. These various liquidcrystal display technologies are described in U.S. Pat. Nos. 5,619,352;5,410,422; and 4,701,028. Conventional approaches do not provide asuitable solution for mass-manufacture fabrication of such acompensation film.

Thus, it can be seen that there is a need for improved apparatus andmethods for fabricating a liquid crystal display compensation filmprovided as a web-fed substrate, where the film comprises twoorthogonally oriented alignment layers.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus and methodfor irradiating a liquid crystal substrate with polarized radiationhaving substantially uniform exposure and a highly uniform direction ofpolarization. It is also an object of the present invention to providean apparatus and method for irradiating a liquid crystal substrate withpolarized radiation having substantially uniform exposure and a highlyuniform direction of polarization. With this object in mind, the presentinvention provides an optical exposure apparatus for forming analignment layer onto either a stationary or web-fed substrate, theapparatus comprising:

-   -   (a) an irradiation apparatus for directing an incident        ultraviolet light toward the substrate at a predetermined        average angle of inclination, over an exposure zone spanning the        full width of the substrate, comprising:        -   (a1) a light source for providing the incident ultraviolet            light;        -   (a2) light directing means for directing the incident            ultraviolet light at the predetermined average angle of            inclination relative to the substrate surface;        -   (a3) light homogenizing means for directing the incident            ultraviolet light along predetermined average angle of            inclination relative to the substrate surface; and    -   (b) a polarizer disposed between the light directing means and        the substrate and rotatably oriented to maintain a principal        axis of polarization having a predetermined orientation with        respect to the web movement direction, the principal axis of        polarization independent of the angle of inclination.

From another aspect the present invention provides a method forfabricating a multilayer stationary or web-fed substrate having a firstliquid crystal film layer aligned along a first optical axis and asecond liquid crystal film layer aligned along a second optical axisorthogonal to the first optical axis, the method comprising:

-   -   (a) applying a first alignment layer to the substrate to form a        multilayer film;    -   (b) irradiating the first alignment layer with incident        ultraviolet light directed through a first polarizer having a        first principal axis of polarization at a first angle relative        to the surface of the multilayer film to provide alignment in a        first direction;    -   (c) applying a second alignment layer to the multilayer film;        and    -   (d) irradiating the second alignment layer with incident        ultraviolet light directed through a second polarizer having a        second principal axis of polarization at a second angle relative        to the surface of the multilayer film, the second angle        orthogonal to the first angle to provide alignment in a second        direction.

It is a feature of the present invention that it directs, over a largeirradiation zone, polarized light having a very uniform direction ofpolarization.

The present invention is capable of directing linearly polarized lightonto a stationary or web-fed substrate, uniform with respect todirection of polarization to within 1 degree, over an exposure zonehaving a width in excess of 1 m.

It is an advantage of the present invention that it provides anirradiation apparatus that can be adapted to expose in one of a numberof polarization directions. Using this feature, a first irradiationapparatus irradiates a substrate with polarized light having a firstpolarization direction and a second irradiation apparatus, similar instructure and composition to the first irradiation apparatus, butarranged at a different angle and having a different number andconfiguration of supporting prisms or louvers, irradiates a substratewith polarized light having a second polarization direction, orthogonalto the first direction.

It is a further advantage of the present invention that it provides UVlight having a highly consistent direction of polarization, whilemaintaining a relatively uniform irradiation over a broad exposure zone.Consistent, uniform polarization direction over this exposure zone isprovided without the need for collimated light.

It is a further advantage of the present invention that it provides asystem capable of continuous, in-line processing of a web-fed substratehaving a plurality of alignment layers that are orthogonally alignedwith respect to each other.

It is a further advantage of the present invention that by allowing theillumination source to be disposed near the substrate surface itprovides improved energy efficiency over prior art photo-alignmentsystems.

It is a further advantage of the present invention that it provides arelatively compact apparatus for irradiation with more uniform dosageover a large exposure zone.

It is a further advantage of the present invention that it allows anumber of irradiation apparatus to be combined in sequence in order toboost processing speed.

It is a further advantage of the present invention that it provides animproved contact-free and dust-free method for forming an alignmentlayer within a liquid crystal substrate.

These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a schematic block diagram showing a system of the presentinvention for processing a multilayer web of liquid crystal compensationfilm;

FIG. 2 is a cross-section view showing media layers of the compensationfilm processed using the apparatus of the present invention;

FIG. 3 is a perspective view showing key geometrical relationships ofstructures within compensation film layers;

FIG. 4 is a perspective view of an irradiation apparatus according tothe present invention;

FIG. 5 is an exploded front view of an irradiation apparatus, showingone configuration of components;

FIG. 6 a is a front view of a irradiation apparatus for a 0-degreeconfiguration;

FIG. 6 b is a cross-sectional side view of an irradiation apparatus fora 0-degree configuration;

FIG. 7 a is a front view of a irradiation apparatus for a 90-degreeconfiguration;

FIG. 7 b is a cross-sectional side view of an irradiation apparatus fora 90-degree configuration;

FIG. 8 a is a front view showing a louver array when in the 90 degreeconfiguration;

FIG. 8 b is a cross-sectional side view of an irradiation apparatus fora 90-degree configuration showing the angular divergence constraints inthe direction of web movement;

FIG. 9 is a front view showing a divertive prism array when in the90-degree configuration;

FIG. 10 a is a representative front view showing the light directingoperation of the louver array;

FIG. 10 b is a representative front view showing the light directingoperation of the louver array in combination with a holographicdiffuser;

FIG. 11 is a representative front view showing the light directingoperation of the divertive prism array FIG.

FIG. 12 a is a perspective top view of a prism array;

FIG. 12 b is a representative front view showing the operation of theprism array to reduce angular divergence;

FIG. 13 is a representative front view showing the combined operation ofthe prism array and the divertive prism array;

FIG. 14 a is a plane view showing a wire grid polarizer array;

FIG. 14 b is a perspective view showing how a wire grid polarizer arrayis assembled, in a preferred embodiment;

FIG. 15 is a top view of a light source alternative, showing an overlappattern using an array of lamps as light source;

FIGS. 16 a and 16 b graphically represent the angular response of aBrewster plate polarizer to light at different incident angles;

FIGS. 16 c and 16 d graphically represent the angular response of wiregrid polarizers and Beilby-layer polarizers for comparison with FIGS. 16a and 16 b;

FIG. 17 illustrates a side view showing the light dispersing property ofa holographic diffuser; and

FIG. 18 graphically illustrates substrate exposure for a 90-degreeconfiguration with and without a light-homogenizing holographicdiffuser.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be directed in particular to elements formingpart of, or in cooperation more directly with the apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art.

The Processing System

Referring to FIG. 1, there is shown a processing apparatus 10 for apreferred embodiment of the present invention for processing a sourceroll 12 of transparent substrate, fed as a web 16 represented as movingfrom left to right in FIG. 1, to provide a finished goods roll 14. Inthe preferred embodiment, finished goods roll 14 is a liquid crystaldisplay compensation film, fabricated as web 16 built up from multiplelayers of material, with the components shown in FIG. 2. These materialsare linear photo-polymerization media (LPP) and liquid crystal polymermedia (LCP).

Referring now to both FIGS. 1 and 2, a clear substrate layer 18 isprovided on source roll 12. In a preferred embodiment, clear substratelayer 18 is made of triacetyl cellulose. An LPP1 layer 22 is added at anLPP1 layer application station 30. A first irradiation station 20 atreats LPP1 layer 22 to provide a predetermined molecular arrangement,crosslinking polymers to obtain optical alignment with a preferred tiltangle. Then, an LCP1 layer 24 is affixed to treated LPP1 layer 22 at anLCP1 layer application station 32. A first curing station 40 a curesLCP1 layer 24 on top of LPP1 layer 22. Next, an LPP2 layer 26 is appliedat an LPP2 layer application station 34. Similarly, LPP2 layer 26 istreated at a second irradiation station 20 b to provide alignment that,in the plane of web 16, is orthogonal to the molecular arrangementprovided to LPP1 layer 22. Finally, an LCP2 layer 28 is applied at anLCP2 layer application station 36 and cured at a second curing station40 b. The manufactured compensation film is then wound onto finishedgoods roll 14.

Using the process shown for processing apparatus 10, continuous,web-based manufacture of a liquid crystal display media or othermaterial can be performed, without the need for cutting individualsheets from a continuous roll. In the processing described, LPP layersare very thin, on the order of approximately 50 nm.

In a preferred embodiment, web 16 is relatively wide, with widthsexceeding 1 m. For treating LPP 1 layer 22 and LPP2 layer 26,irradiation stations 20 a and 20 b provide linearly polarized UV-Birradiation, nominally 280-320 nm. Exposure doses are provided at levelsnear 10-15 mJ/cm². Exposure uniformity within +/−30% is needed in orderto prevent “hot spots” at midpoints in the width of web 16 and to spreadthe exposure energy sufficiently to the sides of web 16. For someapplications, maximum exposure variation much lower than +/−30% may berequired. Most important for proper treatment of LPP1 layer 22 and LPP2layer 26 is providing polarized light having a very consistent directionof polarization, within 1 degree at any point on web 16.

Curing stations 40 a and 40 b apply some form of radiant energy suitablefor curing LCP1 layer 24 and LCP2 layer 28. Preferably, UV-B wavelengthsare avoided to minimize the impact of curing operation on LPP1 layer 22and LPP2 layer 26 once treated. In a preferred embodiment, UV-Awavelengths are used for curing. Some filtering of unwanted wavelengths,such as IR and visible light, is preferred, both at first and secondirradiation stations 20 a and 20 b and at first and second curingstations 40 a and 40 b.

Referring to FIG. 3, there is shown a 3-dimensional geometricrepresentation of LCD layer alignment, relative to an XYZ coordinatesystem. In the upper section of FIG. 3, the orientation of discoticliquid crystal structures 190 is shown for LCP2 layer 28. The lowersection of FIG. 3 shows orientation for LCP 1 layer 24. Optic axes 100and 102 are indicated for LCP1 layer 24 and LCP2 layer 28 respectively.For each LCP1 layer 24 and LCP2 layer 28, alignment processing arrangesdiscotic structures 190 so that optic axes 100 and 102 each have anappropriate tilt angle θ and and/or azimuthal angle φ. Azimuthal angle φis in an XY plane of substrate 192, from the x axis, parallel to web 16within the exposure zone of irradiation station 20 a or 20 b of FIG. 1.Tilt angle θ is in the Z direction, up from the origin. Optic axes 100and 102 should be orthogonal with respect to each other, within atolerance of a few degrees.

Azimuthal angle φ of optic axis 100, 102 is, in part, a factor of dopantin the anisotropic LCP material. Tilt angle θ is, in large part, afactor of the incident light angle, exposure dose, and polarization andcan vary somewhat over a range from θ₁, to θ₂. Obtaining a qualitycompensation film is, then, dependent on maintaining a suitable incidentangle, proper exposure dosage, and uniform polarization characteristics.

In a preferred embodiment, the orthogonal directions of tilt angleorientation are as follows: a 0-degree orientation is providedsubstantially parallel to the direction of web movement; a 90-degreeorientation is provided orthogonal to the 0-degree orientation,substantially in the cross-web direction. With this distinction in mind,subsequent description refers to 0-degree and 90-degree configurations.It must be observed, however, that the apparatus and methods of thepresent invention can be readily adapted to other orthogonal alignmentarrangements. For example, a crossed 45-degree alignment arrangementcould be fabricated, providing orthogonal alignment to multiple layersusing the apparatus and method of the present invention.

Apparatus for Irradiation

Referring to FIG. 4, there is shown an irradiation apparatus 60 as used,with variations, within irradiation stations 20 a and 20 b to apply UVlight over an irradiation zone onto web 16. Irradiation apparatus 60comprises a hood assembly 70 that generates and directs source radiationacross the full width of web 16 and a light conditioning assembly 74 forcontrolling light divergence, for directing light with the desiredincident angle, and for polarizing this source radiation. Within hoodassembly 70, a light source 64 provides source radiation at thepreferred wavelength and power levels. Cooling is provided, such as by acooling tube 66 or other device. Cooling tube 66 could be air- orwater-cooled, for example. An air tube 62 may also be provided forcooling purposes. Cooling tube 66 also provides some amount of UV-C andIR filtering.

As part of light conditioning assembly 74, a prism array 72 is disposedin the radiation path. Prism array 72 helps to reduce the divergenceangle of light along the length of irradiation apparatus 60. An optionalcoating 71 may be provided such as to prevent UV-B reflection, forexample. A louver array 80 is then disposed below prism array 72. As isdescribed subsequently, louver array 80 also helps to reduce thedivergence angle and to direct light in an intended direction toward thesurface of web 16. An optional light homogenizing holographic diffuser85 is disposed below the louver array to direct and homogenize light inthe intended direction toward the surface of web 16. Finally, apolarizer 90 provides the necessary amount of polarization for exposureirradiation, as is also described subsequently.

Light source 64 could be, for example, an Hg medium pressure long arclamp, such as those available from Nordson Corporation, Amherst, Ohio,for example.

FIG. 4 shows one configuration of irradiation apparatus 60. As isdescribed subsequently, different arrangements of support componentssuch as prism array 72, louver array 80, and light homogenizingholographic diffuser 85, are provided for 0- and 90-degreeconfigurations.

Referring to FIG. 5, there is shown an exploded front view ofirradiation apparatus 60. Additional components visible from this viewinclude a filter 82. In a preferred embodiment, filter 82 comprises oneor more fused silica plates, having high reflectance to visible and UV-Alight.

Irradiation station 20 a or 20 b may house more than one irradiationapparatus 60. By using a number of irradiation apparatus 60 in series,first or second irradiation station 20 a or 20 b can provide additionalexposure dosage capacity, effectively providing a wider exposure zone,enabling accelerated processing by processing apparatus 10.

0- and 90-degee Configurations for Irradiation Apparatus 60

As noted above, first irradiation station 20 a may have one or moreirradiation apparatus 60 for providing alignment in one direction.Likewise, second irradiation station 20 b may also have more than oneirradiation apparatus 60 for providing alignment in a second, orthogonaldirection. The deployment of multiple irradiation apparatus 60 unitsincreases the available exposure dosage for treating the LPP medium on aweb-fed substrate. Slightly different configurations of irradiationapparatus 60 are used for the 0-degree and 90-degree configurations usedwithin first and second irradiation stations 20 a and 20 b respectively.However, only minor modifications are needed to adapt irradiationapparatus 60 for the 0-degree or for the 90-degree configuration.

Referring to the front view of FIG. 6 a and corresponding side view ofFIG. 6 b, there is shown the configuration of irradiation apparatus 60for the 0-degree configuration used within first irradiation station 20a. With respect to the view of FIG. 6 a, web 16 movement direction isout from the page. Referring to the corresponding side view of FIG. 6 b,irradiation apparatus 60 is tilted at an angle H relative to the surfaceof web 16. This tilt could be obtained by tilting irradiation apparatus60 or by routing web 16 at an oblique angle relative to irradiationapparatus 60. This arrangement provides exposure light at the optimumincident angle for obtaining 0-degree alignment. A reflector 68 ispositioned along the length of light source 64, collecting andredirecting light emitted from light source 64.

Referring to the front view of FIG. 7 a and corresponding side view ofFIG. 7 b, there is shown the configuration of irradiation apparatus 60for the 90-degree configuration used within second irradiation station20 b. With respect to the view of FIG. 7 a, the direction of movement ofweb 16 is out from the page. Referring to the corresponding side view ofFIG. 7 b, irradiation apparatus 60 is not tilted at an angle relative tothe surface of web 16. However, the angle of incident light must beobliquely diverted in the cross-web direction, as describedsubsequently, either by using a divertive prism array 104 as is shown inFIGS. 7 b and 9, or by changing the angular orientation of louver array80, as is shown in FIG. 8 a.

Reducing Annular Divergence

One function of hood assembly 70 is to minimize the angular divergenceof light incident on polarizer 90. Referring to FIG. 8 b, there is showna side view of irradiation apparatus 60 in the 90-degree configuration.The arrangement of light source 64 and reflector 68 within hood assembly70 effectively constrains the angular divergence of exposure light inthe web travel direction, as indicated by angle F. In a preferredembodiment, angle F is constrained to approximately 45 degrees. Thisangular constraint in the web travel direction applies for both 0-degreeand 90-degree configurations.

Control of angular divergence in the orthogonal direction, across theweb, can be performed by a number of different structures, singly or incombination, depending on the 0- or 90-degree configuration that isused. The simplest means for control of angular divergence is an opaquelouver array 80, with louvers vertically disposed as shown in FIG. 5.With louver array 80 arranged as shown in FIG. 5, angular divergence isconstrained as a factor of louver spacing.

An alternate scheme for control of angular divergence across web 16 usesprism array 72 as shown in FIGS. 12 a and 12 b. Prism array 72 comprisesa sequence of symmetric prisms 73. Each prism 73 has the same dimensionsand is extended lengthwise along the direction of travel of web 16. In apreferred embodiment, height h is approximately 0.5 in., base B isapproximately 1 in angle α₁ is 90 degrees, angles α₂ and α₃ are eachnominally 45 degrees. Other dimensional characteristics could beapplied; prisms 73 could alternately vary in dimension along prism array72. Prisms may have the linear arrangement of FIG. 12 a or some modifiedarrangement, such as staggering of individual prisms 73 to minimizepatterning effects. Prism array 72 is most effective in the 90-degreeconfiguration shown in FIG. 8 a, boosting light efficiency in thisconfiguration.

FIG. 12 b shows the operation of prism array 72 as it cooperates withlight source 64 and reflector 68. Rays R from any point on light source64 are emitted over a broad range of angles. Upon passing through prismarray 72, rays R are refracted to narrow the range of angles. As can beseen in FIG. 12 b, reflective properties of prisms 73 are also employed,in conjunction with reflector 68, to redirect and thereby re-use some ofthe light incident at a range of angles.

The combined effect of controlling angular divergence in the traveldirection of web 16 and across web 16 constrains the angular extent ofthe light cone from any point on light source 64 to polarizer 90. With anarrower range of incident light angles, improved performance ofpolarizer 90 is obtained. However, it is significant to note that, dueto the performance characteristics of polarizer 90 in the preferredembodiment, it is not required that light from light source 64 becollimated.

It should be noted that there can be a number of adaptations of thelouver array 80 arrangement shown in FIG. 5. For example, where anirradiation station 20 a or 20 b comprises multiple irradiationapparatus 60 in series, it may be preferable that, with respect to thedirection of travel of web 16, louvers in one irradiation apparatus 60be offset from louvers in a paired irradiation apparatus 60. Unequallouver spacing can also be used.

Directing Light at Desired Angle of Incidence

For 0-degree alignment at first irradiation station 20 a, tilting hoodassembly 70 provides a desired angle for exposure radiation, directeddown the length of web 16 as is shown in the side view of FIG. 6 b. For90-degree alignment at second irradiation station 20 b, however, thedesired angle of incidence is in an essentially orthogonal direction,across the width of web 16. To divert the exposure radiation from hoodassembly 70 in this direction, there are a number of alternatives. In apreferred embodiment, louver array 80 is disposed as shown in FIG. 10 a.Here, individual louvers 81 are at a suitable angle for directing lightrays R from a source point P toward web 16. With this configuration,each louver 81 has a reflective side 83 and a non-reflective side 84.Rays R at the optimal angle simply pass through louver array 80. Rays Rincident upon reflective side 83 are thereby diverted, where possible,to a more suitable incident angle. Other rays R that cannot be readilydiverted are absorbed at non-reflective side 84. As a result, the rangeof incidence angles obtained is acceptable for alignment in the90-degree orientation. Louvers 81 are spaced to limit the divergenceangle of incident light supplied from light source 64.

An alternative arrangement for diverting exposure irradiation from hoodassembly 70 uses divertive prism array 104 as shown in the front view ofFIG. 9. Referring to FIG. 11, the operation of divertive prism array 104is represented. Here, individual prisms 105 each divert incident lightrays R by refraction, redirecting the light at a suitable incidentangle. Each prism 105 comprises an opaque surface 106 for absorbing raysR that cannot be satisfactorily redirected. Opaque surface 106 istypically an absorptive coating. Dimensionally, prisms 105 may be thesame size or may vary in size across divertive prism array 104.

Referring to FIG. 13, there is shown, from a front view, how divertiveprism array 104 cooperates with prism array 72 in an alternateembodiment. Here, prism array 72 constrains angular divergence of rays Rfrom source point P, as was described with reference to FIG. 12 b. RaysR are then redirected at a suitable incident angle for alignment in thepreferred orientation.

Referring to FIG. 10 b, the operation of louver array 80 as itcooperates with holographic diffuser 85 is illustrated. Rays R fromsource point P are emitted over a broad range of angles. Upon passingthrough louver array 80, rays R are absorbed, transmitted, or reflectedas illustrated in FIG. 10 a With the addition of holographic diffuser 85as illustrated in FIG. 10 b, the holographic diffuser spreads light raysR into cones of radiation. As is subsequently described, the cones caneither be symmetric with apex angle of ε, or a more general ellipticalcone. By selecting a maximum diffusion angular spread ε in combinationwith suitable means of angular divergence reduction, the overall lightdivergence is maintained within acceptable limits. A typical maximumangular light diffusion angle selected for a holographic diffuser wouldbe 20 degrees. Angular divergence reduction may be louvers asillustrated in FIG. 10 b, prisms only as illustrated in FIG. 9, or acombination of prisms and louvers as illustrated in FIG. 8 a.

Homogenized Light for Uniform and Efficient Exposure in a Compact Space

Referring to FIG. 18, there is shown a representation of the uniformityof exposure 140 that may be expected from a 90-degree web-fed system asillustrated in FIG. 8 a without a holographic diffuser 85. Theuniformity of exposure is measured at the substrate 16. Although thisFigure is shown for a 90-degree system, the same discussion pertains tothe 0-degree system. Two major variations exist; the first is the slowdecrease in average dose from a maximum in the middle of the substrateto a minimum at the edges of the substrate; the other is a rapidlyvarying dose over small distances. This dose variation generally has aspatial periodicity related to the periodicities of the componentsdirectly above the substrate 16, i.e., louvers and/or prisms. The firstslow variation in dose from center to edge is the result of the finiteextent of the light source 64 relative to the width of web-fed substrate16. The second variation is a result of the individual louvers andprisms imparting small-scale changes to transmitted light. The firstvariation may be reduced through suitable selection of lamp lengthrelative to the maximum web widths. Alternatively, the first variationmay be reduced through by many means: altering individual spacings orsizings of prisms and louvers, changing reflectances or transmittancesof louvers and prisms, adjusting the exposure time along the width ofthe web, or by increasing the distance of the substrate 16 from thelight conditioning assembly 74. The second rapid variation in exposurecannot be as easily measured and controlled by the methods that may beused to address the slow variation. Referring again to FIG. 18,introducing a holographic diffuser 85 as illustrated in FIG. 8 a mayvirtually eliminate the small-scale exposure variation 142.

The introduction of a holographic diffuser 85 between the polarizer 90and louver array 80 improves the homogenization of the irradiated lightsuch that exposure will be more uniform and reduces the slope with whichthe exposure changes occur. This improvement in exposure uniformitypertains to both stationary and moving substrates.

The use of holographic diffusers to homogenize light is described in theliterature (e.g., Baturin et al. “Application of Holographic Diffusersto Improve Light Uniformity of Source with Carbon Fiber Cathodes.”) Theholographic diffuser also permits the apparatus to remain more compactthan it otherwise would be without a holographic diffuser.

A holographic diffuser is a surface relief hologram. They also may beknown by registered names; for example, Light Shaping Diffuser (PhysicalOptics Corporation; Torrance, Calif.) and Tailored Micro Diffusers(Wavefront Technology Inc., Paramount, Calif.). Holographic diffusersmay be embossed, molded or etched into deformable material such asacrylic, polycarbonate, other plastics, glass or fused silica. Themethod of inexpensively fabricating holographic diffusers from materialsother than plastic is a more recent manufacturing development, forexample, as illustrated by the contact molding method described in U.S.Pat. No. 6,352,759. Holographic diffusers are characterized bytransmission efficiency; they offer superior optical transmission,typically up to 92% in visible light, greatly exceeding the lightthroughput characteristics of a conventional ground-glass diffuser.Diffusers are also characterized by a specific tailored output angulardistribution and can be designed to disperse light into cones from lessthan 1 degree to >90°. Referring to FIG. 17, incident rays R incident onholographic diffuser 85 are spread into cones of radiation 87. Thesimplest type of holographic diffuser will approximately spread lightinto a circularly symmetric cone with maximum divergence apex angle ofε. Alternatively, the light “cone” can be in elliptical form, so thatdivergence is greater along a major-axis and less along a minor-axis;i.e., ε may have different values depending on the cross-section.Holographic diffusers are also characterized by a tendency to introducea birefringence or depolarization component to the incident light.

For the requirement of UV alignment on a stationary or moving web-fedsubstrate with high light transmission, uniformity of dosage, anduniformity of polarization, the preferred embodiment for a holographicdiffuser is one made from fused silica for high UV transmittance.Additionally, as illustrated in FIGS. 6 a and 6 b, the holographicdiffuser 85 is preferably placed between the polarizer 90 and the prismarray 72. Referring to FIGS. 8 a and 8 b, the holographic diffuser 85 ispreferably placed between the polarizer 90 and the louver array 80. Theholographic component divergence angle would be chosen with angulardivergence small enough so as not to deleteriously affect the angleof-incidence properties required of the 0 and 90 degree systems. For the0-degree alignment apparatus, angular divergence of the elliptical“cone” along the substrate-travel direction would typically be less than20 degrees and between 10 and 60 degrees in the cross-track direction.For the 90-degree alignment apparatus, angular divergence of theelliptical “cone” along the cross-track direction would typically beless than 20 degrees, and along the substrate-travel direction, thedivergence would typically be between 10 and 100 degrees. Limitedavailability of commercial elliptical holographic diffusers thattransmit in the UV limit choices to circular diffusers that typicallyfunction with lower angular divergence ranges.

Configuration of Polarizer 90

Referring back to FIGS. 16 a through 16 d, the significance ofmaintaining a uniform principal axis of polarization 126 at polarizer 90can be appreciated. When polarizer 90 has this property, incident lightcan vary over a range of angles without affecting the axis ofpolarization that is provided.

As a general rule, polarizer 90 works best when used with light having areduced divergence angle and when placed in close proximity to web 16.Referring to FIGS. 14 a and 14 b, there is shown a plane view and anexploded view, respectively, of polarizer 90 in a preferred embodiment.Polarizer segments 91 are wire grid polarizers, typically three inchsquare in a preferred embodiment, tiled together as shown. Polarizer 90employs a number of polarizer segments 91, preferably disposed at anangle relative to the movement of web 16. The angular offset compensatesfor possible streaking effects from boundaries between individualpolarizer segments 91. A grid frame 96 and a cover frame 94 are used tohold polarizer segments 91 in place, sandwiched between masks 92.

With the arrangement of FIGS. 14 a and 14 b, polarizer 90 is rotatable.That is, the complete polarizer 90 assembly can be rotated toeffectively change the principal axis of polarization with respect tothe exposure zone of web 16. Alternately, individual polarizer segments91 can be rotated so that polarizer 90 can be adapted to provide a rangeof angles.

Wire grid polarizers are used as polarizer segments 91 in a preferredembodiment due to their capability to withstand high temperatures andsuitable polarization characteristics. Generally, polarization of 3:1 orbetter is acceptable; 5:1 provides a preferred ratio for alignment. Wiregrid polarizers offer advantages due to their inherent heat resilienceand reduced thickness. Because these devices are generally lesssensitive to angular variation than are Brewster plate polarizers orother polarizer devices, wire grid polarizers do not require collimatedlight. Additionally, because these devices are reflective, some amountof unused light can be reflected back and, potentially, re-used. Wiregrid polarizers can be easily oriented to provide 0-degree and 9° degreealignment simply by rotating the wire direction of polarizer segments 91within the plane of polarizer 90.

Other types of polarizers 90 could be used, provided they are capable ofoperating under relatively high temperature conditions. Brewster platesmade of fused silica or quartz could also be used for alignment in themovement direction of web 16, however, size and weight constraints limitthe feasibility of such a solution for large scale irradiation. It wouldalso be difficult to maintain a uniform principal axis of polarizationwhen using Brewster plate devices. Moreover, as is noted in thebackground section of this application, a Brewster plate polarizer wouldnot be well-suited for cross-web irradiation, particularly where along-arc lamp is used as light source 64. Interference-type polarizersthat also employ Brewster plate principles could alternately be used;however, these polarizers are not dimensionally advantageous and arealso poorly suited for cross-web irradiation.

An alternate type of polarizer segment 91 could employ a Beilby-layerpolarizer, such as those available from Sterling Optics, Inc. A matrixarrangement similar to that shown in FIGS. 14 a and 14 b would also bepracticable when using Beilby-layer devices. The corresponding polarizersegments 91 could be larger when using these devices than with wire gridpolarizers.

With the 90-degree alignment configuration of FIGS. 7 a and 7 b,polarizer 90 is optimally positioned parallel to the surface of web 16within the exposure zone. With the 0-degree alignment configuration ofFIGS. 6 a and 6 b, polarizer 90 may be parallel to the base of hoodassembly 70, parallel to web 16, or at some intermediate angle. Theremay be some advantages to using angles that maximize levels ofirradiance and provide highly uniform polarization, particularly withthe O-degree arrangement. As was described with reference to FIGS. 16 cand 16 d, wire grid and Beilby-layer polarizers maintain a uniformprincipal axis of polarization for light over a range of incidentangles. Because of this, polarizer 90, when fabricated using thesedevices, can be tilted about an axis parallel to the length of lightsource 64 without noticeable effect on uniformity of polarizationdirection. Tilt angles of up to about 30 degrees with respect to thesurface of web 16 can be used without affecting the performance ofpolarizer 90.

Alternate embodiments for Light Source 64

In a preferred embodiment, light source 64 is a medium-pressure mercuryarc lamp. These devices can have a power input approaching or exceeding400 watts/inch of lamp length and have a favorably long lamp life. Thisapproach provides the advantages of using a single bulb that hasrelatively long useful life (1,000 hours, nominal) and can be readilychanged with minimal disruption to equipment.

Alternatives for light source 64 include mercury short arc lamps,typically having 10-15 kW input. However, mercury short arc lamps can bemore expensive and typically have a shorter useful life thanmedium-pressure arc lamps. Referring to FIG. 15, there is shown anarrangement using multiple light sources 164 a, 164 b, 164 c, 164 d, 164e, and 164 f. In order to span the width of web 16, light sources 164 a,164 b, 164 c, 164 d, 164 e, and 164 f are grouped, staggered to leave noperceptible gaps or boundaries.

Embodiments for Filter 82

Filtering of the UV light helps to attenuate harmful wavelengths and toremove unwanted light and heat from the system. UV-C light (200-270 nm),for example, appears to be detrimental to LPP crosslinking needed foralignment. One method for removing UV-C wavelengths employs an absorbingplate of WG 280 for example. UV-A, visible, and infrared light areconsidered superfluous, however, to minimize heating and other unwantedside effects, the preferred embodiment uses an optional filter 82 toremove this unwanted light from the system. Additional filtering is alsoprovided near light source 64 by cooling tube 66, particularly wherewater is used, as noted above.

In a preferred embodiment, filter 82 follows louver array 80 and prismarray 72 in the optics path. With this arrangement, filter 82 acceptsincident light over a more controlled angle of divergence. However,filter 82 could-be positioned at alternate points in the optics path.

Alternative Embodiments for Processing Apparatus 10

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention as described above, and as noted in the appended claims, by aperson of ordinary skill in the art without departing from the scope ofthe invention. For example, while hood assembly 70 is at right angles tothe direction of web 16 movement in the preferred embodiment, otherarrangements are possible. For example, hood assembly 70 could bedisposed at a diagonal relative to web 16. Other types of light sourcescould be employed, providing that a generally uniform irradiation isobtained.

Irradiation apparatus 60 can also be adapted to suit requirements forobtaining the desired direction of polarization and pretilt. In apreferred embodiment, irradiation apparatus 60 used for the 90-degreeconfiguration uses both prism array 72 and louver array 80. However, inthe 0-degree configuration, irradiation apparatus 60 does not use louverarray 80. It can be seen that other arrangements are possible, so thateach irradiation apparatus 60 can be equipped with either louver array80 or prism array 72 or with both louver array 80 and prism array 72.For 90-degree configurations, louver array 80 may be angled andconfigured as described earlier with reference to FIG. 10. Alternately,the combination using both prism array 72 and divertive prism array 104could be used.

In the preferred embodiment, LPP layers are orthogonally aligned in thedirection of travel of web 16 and in the cross-web direction.Alternately, a different set of orthogonal angles could be used. Forexample, alignment angles could be at 45-degrees with respect to theedges of web 16.

Alternate system arrangements for processing apparatus 10 could employ asingle irradiation apparatus 60 for processing a roll of media with onealignment layer, aligned in a 45-degree orientation. A second alignmentlayer having 135-degree alignment could be fabricated separately usingthe same irradiation apparatus 60. Then, both orthogonal layers could belaminated together in an orthogonal arrangement.

Additional light conditioning components could be used in the opticspath, such as for additional filtering. Optionally, a diffuser, such asa holographic diffuser, could be employed for improving uniformity ofirradiance.

It can be seen that irradiation apparatus 60 of the present inventionprovides a compact, energy-efficient device for exposing a wide-areasubstrate with light radiation having a uniform polarization direction.Adaptable for providing incident light in orthogonal directions,irradiation apparatus 60 can be advantageously deployed in aroll-to-roll manufacturing environment for fabricating web-fedmultilayered liquid crystal media having orthogonally oriented alignmentlayers.

Thus, what is provided is and apparatus and method for processing aliquid crystal display compensation film provided as a web-fedsubstrate.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   10 Processing apparatus-   12 Source roll-   14 Finished goods roll-   16 Web-   18 Clear substrate layer-   20 a First irradiation station-   20 b Second irradiation station-   22 LPP 1 layer-   24 LCP1 layer-   26 LPP2 layer-   28 LCP2 layer-   30 LPP 1 layer application station-   32 LCP1 layer application station-   34 LPP2 layer application station-   36 LCP2 layer application station-   40 a First curing station-   40 b Second curing station-   60 Irradiation apparatus-   62 Air tube-   64 Light source-   66 Cooling tube-   68 Reflector-   70 Hood assembly-   71 Coating-   72 Prism array-   73 Prisms-   74 Light conditioning assembly-   80 Louver array-   81 Louver-   82 Filter-   83 Reflective side-   84 Non-reflective side-   85 Holographic diffuser-   87 Cones of radiation-   90 Polarizer-   91 Polarizer segment-   92 Mask-   94 Cover frame-   96 Grid frame-   100 Optic axis-   102 Optic axis-   104 Divertive prism array-   105 Prism-   106 Opaque surface-   120 Reflected beam-   122 Transmitted beam-   124 Incident beams-   126 Principal axis of polarization-   128 Reference direction-   130 Meridional plane-   132 Brewster plate polarizer-   134 Wire grid polarizer-   140 Exposure without holographic diffuser-   142 Exposure with holographic diffuser-   164 a Light source-   164 b Light source-   164 c Light source-   164 d Light source-   164 e Light source-   164 f Light source-   190 Discotic structures-   192 Plane of substrate

1. An optical exposure apparatus for forming an alignment layer onto asubstrate, the apparatus comprising: (a) an irradiation apparatus fordirecting an incident ultraviolet light toward said substrate at apredetermined average angle of inclination, over an exposure zonespanning a full width of said substrate, comprising: (a1) a light sourcefor providing said incident ultraviolet light; (a2) light directingmeans for directing said incident ultraviolet light at saidpredetermined average angle of inclination relative to said substrate;(a3) a filter; (a4) a polarizer wherein said polarizer is rotatablyoriented to maintain a principal axis of polarization, and saidprincipal axis of polarization is independent of said angle ofinclination; and (a5) a holographic light diffuser for homogenizing saidincident ultraviolet light on said substrate at said predeterminedaverage angle of inclination relative to said substrate.
 2. An opticalexposure apparatus according to claim 1 wherein said substrate isweb-fed.
 3. An optical exposure apparatus according to claim 1 whereinsaid irradiation apparatus further comprises a reflector partiallysurrounding said light source.
 4. An optical exposure apparatusaccording to claim 1 wherein said light source comprises a plurality oflamps.
 5. An optical exposure apparatus according to claim 1 whereinsaid light source comprises a long-arc UV lamp.
 6. An optical exposureapparatus according to claim 2 wherein said first principal axis ofpolarization is parallel to a direction of movement of said substrate.7. An optical exposure apparatus according to claim 1 wherein saidpolarizer comprises a plurality of wire grid polarizers.
 8. An opticalexposure apparatus according to claim 1 wherein said polarizer comprisesa Beilby-layer polarizer.
 9. An optical exposure apparatus according toclaim 2 wherein said irradiation apparatus is disposed at apredetermined tilt angle, relative to said exposure zone, along a tiltaxis that extends across said width of said moving substrate.
 10. Anoptical exposure apparatus according to claim 1 wherein said polarizercomprises a matrix of wire grid polarizers, wherein each of said wiregrid polarizers is selectively oriented at a first angle for providingpolarization along said first principal axis of polarization or orientedat a second angle for providing polarization along said second principalaxis of polarization.
 11. An optical exposure apparatus according toclaim 1 wherein a surface of said polarizer that receives said incidentlight is angularly disposed relative to a surface of said substratewithin said exposure zone within a range from parallel to 30 degrees.12. An optical exposure apparatus for forming an alignment layer onto asubstrate, the apparatus comprising: (a) an irradiation apparatus fordirecting an incident ultraviolet light toward said substrate at apredetermined average angle of inclination, over an exposure zonespanning a full width of said substrate, comprising: (a1) a light sourcefor providing said incident ultraviolet light; (a2) a prism array andlouver array for directing said incident ultraviolet light at saidpredetermined average angle of inclination relative to said substrate;(a3) a plurality of louvers for guiding said incident ultraviolet lighttoward said prism array: and (a4) a holographic light diffuser forhomogenizing said incident ultraviolet light on said substrate at saidpredetermined average angle of inclination relative to said substrate.13. An optical exposure apparatus according to claim 12 wherein apolarizer is disposed between said prism array and said substrate androtatably oriented to maintain a principal axis of polarization having apredetermined orientation with respect to a web movement direction, saidprincipal axis of polarization independent of said angle of inclination.14. An optical exposure apparatus according to claim 12 wherein saidsubstrate is web-fed.
 15. An optical exposure apparatus according toclaim 12 further comprising a filter disposed between said prism arrayand said substrate.
 16. An optical exposure apparatus for forming analignment layer onto a substrate, the apparatus comprising: (a) anirradiation apparatus for directing an incident ultraviolet light towardsaid substrate at a predetermined average angle of inclination, over anexposure zone spanning a full width of said substrate, comprising: (a1)a light source for providing said incident ultraviolet light; (a2) aplurality of louvers for directing said incident ultraviolet light atsaid predetermined average angle of inclination relative to saidsubstrate, wherein each of said louver has a reflective side and anopaque side, and wherein a long axis of each of said louver is alignedparallel to a web movement direction; (a3) a filter disposed betweensaid substrate: and (a4) a holographic light diffusing means forhomogenizing said incident dosage on said substrate surface at saidpredetermined average angle of inclination relative to said substrate.17. An optical exposure apparatus according to claim 16 wherein apolarizer is disposed between said plurality of louvers and saidsubstrate and rotatably oriented to maintain a principal axis ofpolarization having a predetermined orientation, wherein said principalaxis of polarization is independent of said angle of inclination.
 18. Anoptical exposure apparatus according to claim 16 wherein said pluralityof louvers are tilted at an oblique angle with respect to a surface ofthe substrate.
 19. An optical exposure apparatus for applying radiantenergy onto a substrate, the apparatus comprising: (a) an irradiationapparatus for directing an incident ultraviolet light toward saidsubstrate at a predetermined average angle of inclination, over anexposure zone spanning a full width of said substrate, comprising: (a1)a light source for providing said incident ultraviolet light; (a2) afirst prism array for providing a reduced divergence angle to saidincident ultraviolet light; (a3) a second prism array for directing saidincident ultraviolet light having said reduced divergence angle at saidpredetermined average angle of inclination relative to said substrate;(a4) a filter disposed between said substrate; and (a5) a holographiclight diffuser for homogenizing said incident dosage on said substrateat said predetermined average angle of inclination relative to thesubstrate.
 20. An optical exposure apparatus according to claim 19wherein a polarizer is disposed between said second prism array and thesubstrate and rotatably oriented to maintain a principal axis ofpolarization having a predetermined orientation, wherein said principalaxis of polarization independent of said angle of inclination.
 21. Anoptical exposure system for fabricating a multilayer film on a web-fedsubstrate, wherein a first liquid crystal film layer is aligned along afirst optical axis and a second liquid crystal film layer is alignedalong a second optical axis, wherein said second optical axis isorthogonal to said first optical axis, said optical system comprising:(a) a first irradiation apparatus for directing a first incidentultraviolet light at a first predetermined average angle of inclinationtoward said substrate, over an exposure zone spanning a full width ofthe substrate, comprising: (a1) a first light source for providing saidfirst incident ultraviolet light; (a2) first light directing means fordirecting said first incident ultraviolet light at said firstpredetermined average angle of inclination relative to said substrate;(a3) a first holographic light diffuser for homogenizing said incidentdosage on said substrate at said predetermined average angle ofinclination relative to the substrate surface; (b) a first polarizer forpolarizing said first incident ultraviolet light in a first uniformpolarization direction over said exposure zone, said first polarizercomprising tiled polarization components rotated to a first polarizationaxis angle; (c) a second irradiation apparatus for directing a secondincident ultraviolet light at a second predetermined average angle ofinclination toward said substrate, over said exposure zone spanning thefull width of the substrate, comprising: (c1) a second light source forproviding said second incident ultraviolet light; (c2) second lightdirecting means for directing said second incident ultraviolet light atsaid second predetermined average angle of inclination relative to saidsubstrate; (c3) a second holographic light diffuser means forhomogenizing said incident dosage on said substrate at saidpredetermined average angle of inclination relative to said substrate;and (d) a second polarizer for polarizing said second incidentultraviolet light in a second uniform polarization direction over saidexposure zone, said second polarizer comprising tiled polarizationcomponents rotated to a second polarization axis angle.
 22. A method forfabricating a multilayer web-fed substrate having a first liquid crystalfilm layer aligned along a first optical axis and a second liquidcrystal film layer aligned along a second optical axis orthogonal tosaid first optical axis, the method comprising: (a) applying a firstalignment layer to said substrate to form a multilayer film; (b)irradiating said first alignment layer with incident ultraviolet lightdirected through a first polarizer having a first principal axis ofpolarization at a first angle relative to the surface of the multilayerfilm to provide alignment in a first direction; (c) applying a secondalignment layer to said multilayer film; (d) irradiating said secondalignment layer with incident ultraviolet light directed through asecond polarizer having a second principal axis of polarization at asecond angle relative to the surface of the multilayer film, said secondangle orthogonal to said first angle to provide alignment in a seconddirection; and wherein the step of irradiating said first alignmentlayer comprises the step of disposing a holographic diffuser between asource of illumination and said substrate.
 23. A method for fabricatinga multilayer web-fed substrate according to claim 22 wherein the step ofirradiating said first alignment layer comprises the step of disposingan array of prisms between a source of illumination and the substrate.24. A method for fabricating a multilayer web-fed substrate according toclaim 22 wherein the step of irradiating said first alignment layercomprises the step of disposing an array of louvers between a source ofillumination and the substrate.
 25. A method for fabricating amultilayer web-fed substrate according to claim 22 wherein said firstpolarizer is an array comprising individual wire grid polarizersegments.
 26. A method for fabricating a multilayer web-fed substrateaccording to claim 22 wherein said first polarizer is an arraycomprising individual Beilby-layer polarizer segments.
 27. A method forfabricating a multilayer web-fed substrate according to claim 22 whereinthe step of irradiating said second alignment layer comprises the stepof using a first array of prisms to provide a reduced divergence angleand using a second array of prisms to bend the light path to provideincident light at a predetermined angle.
 28. An optical exposureapparatus for forming an alignment layer onto a web-fed substrate, theapparatus comprising: (a) an irradiation apparatus for directing anincident ultraviolet light toward said substrate at a predeterminedaverage angle of inclination, over an exposure zone spanning the fullwidth of said substrate, comprising: (a1) a light source for providingsaid incident ultraviolet light; (a2) a hood assembly for directing saidincident ultraviolet light at said predetermined average angle ofinclination relative to said substrate surface; and (a3) a holographiclight diffusing means for homogenizing said incident dosage on saidsubstrate.